Lift drill into evals/ at 013fcb8b7dbefd6d3fa4653493e5d2ec8e7f985b

rsync of obra/drill@013fcb8b7d into superpowers/evals/, excluding
.git/, .venv/, results/, .env/, __pycache__/, *.egg-info/,
.private-journal/.

The drill repo is unaffected by this commit; archival is a separate
manual step after this PR merges.

Source SHA recorded at evals/.drill-source-sha for divergence
detection.

evals/tests/test_stats.py

@@ -0,0 +1,54 @@
+"""Tests for Wilson score confidence interval."""
+
+from __future__ import annotations
+
+from drill.stats import wilson_ci
+
+
+class TestWilsonCI:
+    def test_all_pass(self) -> None:
+        lo, hi = wilson_ci(10, 10)
+        assert lo > 0.69
+        assert hi == 1.0 or hi > 0.99
+
+    def test_all_fail(self) -> None:
+        lo, hi = wilson_ci(0, 10)
+        assert lo < 0.01 or lo == 0.0
+        assert hi < 0.31
+
+    def test_half_pass(self) -> None:
+        lo, hi = wilson_ci(5, 10)
+        assert 0.18 < lo < 0.25
+        assert 0.75 < hi < 0.82
+
+    def test_zero_total(self) -> None:
+        lo, hi = wilson_ci(0, 0)
+        assert lo == 0.0
+        assert hi == 0.0
+
+    def test_single_pass(self) -> None:
+        lo, hi = wilson_ci(1, 1)
+        assert lo > 0.0
+        assert hi <= 1.0
+
+    def test_single_fail(self) -> None:
+        lo, hi = wilson_ci(0, 1)
+        assert lo == 0.0 or lo >= 0.0
+        assert hi < 1.0
+
+    def test_large_sample(self) -> None:
+        lo, hi = wilson_ci(80, 100)
+        assert 0.70 < lo < 0.75
+        assert 0.85 < hi < 0.90
+
+    def test_passed_greater_than_total_clamped(self) -> None:
+        lo, hi = wilson_ci(12, 10)
+        assert lo > 0.0
+        assert hi <= 1.0
+
+    def test_returns_tuple_of_floats(self) -> None:
+        result = wilson_ci(5, 10)
+        assert isinstance(result, tuple)
+        assert len(result) == 2
+        assert isinstance(result[0], float)
+        assert isinstance(result[1], float)